1. Field of the Invention
The present invention relates generally to silicon resin fouling release coatings, and more specifically to fluorinated silicone resin antifoulant compositions.
2. Background of the Invention
Aquatic animal and plant organisms such as barnacles, oysters, ascidians, polyzoans, serupulas, sea lettuces and green layers adhere and grow on the surface of marine structures, resulting in various damage. For example, the aquatic organisms can adhere onto the bottom of a ship to increase the frictional resistance between the ship body and water. The increased resistance results in increased fuel costs. Some industrial plants use sea water for cooling. Fouling of intake pipes by aquatic animals and plants can hinder the induction of cooling water resulting in a drop in cooling effectiveness. A wide range of marine structures such as undersea construction, piers, buoys, harbor facilities, fishing nets, ships, marine tanks, water conduit raceway tubes of power plants and coastal industrial plants are affected.
Marine organisms initially are attracted to and subsequently attach to a surface by chemical and physical means. Biopolymers such as polypeptides and polysaccharides comprise the outermost layer of marine organisms, and in some cases the marine organism exudes a "glue" (which is typically comprised of similar material) by which it attaches to a substrate. Biopolymers are very polar, and initial physical attachment to a substrate is facile when the substrate contains polar groups to which these biopolymers can hydrogen-bond. Further chemical attachment can take place by reactions between the biopolymers and a substrate. A hydrophobic surface is one which contains very little to no polar groups; thus, a hydrophobic surface expresses very few "toeholds" for marine organisms to adhere. The only type of attraction would be Van der Waals attractions, which are very weak.
Various antifouling compositions have been developed to prevent the adherence of the aquatic organisms. Toxicants containing copper, tin, arsenic and mercury and various compounds of the same have been proposed. Other proposed materials include strychnine, atropine, oxides of zinc, lead and antimony, creosote, phenol, metallic silver, iodine, bromine and mixtures of iron, copper and zinc powders. Of the above compositions, only three groups, namely cuprous, tin and mercury compounds, have been useful, even to a limited degree. However, even the effective compositions have disadvantages. These compositions prevent fouling by a toxic mechanism. Effectiveness of the compositions requires that a lethal concentration of poison be maintained in the water immediately adjacent to the surface of the marine structure Eventually, the poison is completely leached into the water and the composition is exhausted and must be replaced. Further, the poisons are toxic to humans and can be a major source of pollution in busy harbors and in waterways.
Fouling release coatings, that is coatings which do not allow organisms to adhere to the marine body surface have been proposed as alternatives to the toxicity-based anti-foulants. Lampe et al., U.S. Pat. No. 4,861,670 discloses a silicone fouling release composition. Silicones are based on polymers comprised of a backbone of silicon-oxygen-silicon atoms linked together. They are thus different chemically from organic materials which are based on polymers comprised of a backbone of carbon-to-carbon atoms linked together. It is this difference--the silicon-oxygen linkage--which accounts for the unique properties of silicones. The silicone based linkage imparts both high temperature resistance and stability toward many deteriorating influences such as ozone, chemicals, environment and radiation. The silicones are generally comprised of linear silicone resins which possess two or more crosslinking sites. Typically the sites are hydroxy functional. The silicones are cured by crosslinking at the sites. Typically, the crosslinking is achieved through the addition of a crosslinking agent such as a multi-functional alkoxy silane such as alkylmethoxy silane and tetraethoxy silane.
However, the silicones have certain disadvantages. First, while typical silicone surface energy at 20-24 dynes/cm is marginally low enough for an effective release coating, other materials are known with lower surface energy that could be more effective as fouling release coatings. Second, in time, hydrophilic sites migrate to the surface of the silicone coatings to decrease the coating surface hydrophobicity. Third, silicones do not provide durable, long-life coatings because of low tear strength. Fourth, residual by products from resin syntheses and performance additives in the silicones contribute to marine pollution by leaching into the water.
There remains a need for a coating that has effective fouling release properties, is hydrophobic, durable and non polluting.